Method for etching chromium film formed on substrate

ABSTRACT

A method for etching a chromium film includes the steps of forming a resist having a phenol novolak resin as a principal chain on the chromium film formed on a substrate, and etching the chromium film using an etchant while stripping off the resist from the chromium film using an etchant containing nitric acid. A second method for etching a chromium film includes the steps of forming an aluminum film or an aluminum alloy film on the chromium film formed on a substrate, forming a resist having a predetermined pattern on the aluminum film or the aluminum alloy film, etching the aluminum film or the aluminum alloy film and the chromium film using phosphoric acid, and removing by etching the aluminum film or the aluminum alloy film using phosphoric acid containing nitric acid after removing the resist.

This application is a continuation of application Ser. No. 07/249,905filed Sep. 27, 1988, now U.S. Pat. No. 5,007,984.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method for etching achromium film formed on a substrate, and more particularly, to a methodfor etching a chromium film suitable for a multilayer interconnectionbody in which a film is further formed on the chromium film.

2. Description of the Prior Art

FIGS. 1A, 1B, and 1C are cross sectional views for explaining an exampleof a conventional method for etching a chromium film. FIG. 1A shows astate before etching a chromium film. In FIG. 1A, a chromium film 2 isformed on a substrate 1, and a resist pattern 3 is formed on thechromium film 2. The resist pattern 3 can be formed using aphotolithographic technique ordinarily used. The chromium film 2 is thendipped in an etchant having the composition of 17 g of ammonium cerium(IV) nitrate, 5 cc of perchloric acid and 100 cc of water. This etchantis a chromium etchant used in patterning a chromium film, which isdescribed, for example, by Naraoka and Nihei in the "Photo-Etching andFine Structure Processing", Sogodenshi Shuppansha, issued May 10, 1977.The chromium film 2 is oxidized in the etchant, to be ions. The ionsdissolve in the etchant. At this time, a portion coated with the resistpattern 3 does not come into contact with the etchant, so that thechromium film under the resist pattern 3 does not dissolve. Thus, thechromium film 2 in a portion where the resist pattern 3 is not provideddissolves, as shown in FIG. 1B. Then, as shown in FIG. 1C, the resistpattern 3 is removed by plasma ashing, so that a pattern of the chromiumfilm 2 is obtained.

An etching end surface of the chromium film obtained by such aconventional method is approximately vertical, as shown in FIG. 1C.Therefore, when a film is further formed on this chromium film, theheight of a portion corresponding to an underlayer rapidly changes, sothat crystalline characteristics of the film in this portion are liableto change. Consequently, defects occur in a film formed on the chromiumfilm, whereby the reliability is decreased.

In order to solve such a problem, Japanese Patent Laying-Open GazetteNo. 111366/1983 discloses a method for etching a metal film formed on asubstrate in a tapered shape. FIGS. 2A, 2B and 2C are cross sectionalviews for explaining the method disclosed in this gazette. FIG. 2A showsa state in which a resist is formed on a chromium film 12 duly formed ona substrate 11 and the resist is removed after the first etching. FIG.2B shows a state in which a resist 14 is formed on the chromium film 12shown in FIG. 2A so as to be of a pattern narrower than the pattern ofthe chromium film 12. The chromium film 12 is then etched while being incontact with a chromium etchant in this state. On this occasion, thechromium film 12 is not completely etched but the etching thereof isterminated in the halfway stage. Consequently, a cross section of thechromium film 12 can be made to have a gentle shape as shown in FIG. 2C.Therefore, the change in crystalline characteristics of the film formedon the chromium film can be made small, so that occurrence of defectscan be suppressed and the reliability can be increased.

However, in this conventional method, the process for forming the resistpattern is required two or more times, so that the manufacturing processbecomes complicated and the manufacturing cost becomes high.

Additionally, in this method, the second or the subsequent etching mustbe terminated halfway. Therefore, it is necessary to determine the timepoint when etching is terminated. However, it is difficult to determinethe time point when etching is terminated, so that it is difficult tocontrol a tapered shape of an etching end surface.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for etching achromium film by which a side surface of the chromium film can betapered in a simple process.

Another object of the present invention is to provide a method by whicha chromium film can be etched such that a side surface thereof makes anangle with a predetermined slope.

Another object of the present invention is to provide a method by whicha chromium film can be etched in a desired shape in a reproduciblemanner by controlling the shape of a side surface of the chromium film.

The inventors of the present invention have found that a coating film isformed on a chromium film and the chromium film is etched whiledecreasing the contact area of the coating film with the chromium film,so that an etching end surface, i.e., a side surface of the chromiumfilm can be formed in a tapered shape.

More specifically, an etching method according to the present inventioncomprises the steps of forming a coating film on a chromium film formedon a substrate, and etching the chromium film using an etchant whiledecreasing the contact area of the chromium film with the coating film.According to the present invention, during etching, the contact area ofthe chromium film with the coating film is decreased as the etchingprogresses. Therefore, the closer the position of the chromium filmcomes to the coating film, the larger the degree of etching of thechromium film becomes. As a result, the etching end surface is inclined,to have a tapered shape.

In accordance with a preferred aspect of the present invention, theetching method comprises the steps of forming a resist having apredetermined pattern on the chromium film formed on the substrate, andetching the chromium film using an etchant while stripping off theresist from the chromium film. In this aspect, as the etchingprogresses, the resist is stripped off from the chromium film, so thatthe contact area of the chromium film with the coating film, i.e., theresist is reduced. Such stripping of the resist from the chromium filmmay be achieved by physically stripping off the resist from the chromiumfilm or stripping off the resist from the chromium film by chemicalmeans. In a preferred manner of this aspect, the resist is stripped offfrom the chromium film using the etchant. Such stripping can be achievedby selecting an etchant which corrodes the resist in the interface ofthe resist and the chromium film. If and when a resist having a phenolnovolak resin as a principal chain is used, which resist is generallyused, the resist can be stripped off by using an etchant includingnitric acid. In the case of the resist generally used, the chromium filmis etched while stripping off the resist from the chromium film using anetchant containing 2 or more mol/liter or nitric acid, so that anetching end surface of the chromium film can be tapered. As will bedescribed in the following embodiment, the concentration of nitric acidin the etchant can be suitably selected depending on the type of theresist and the temperature of the etchant. An angle of the tapered shapeof the etching end surface of the chromium film can be controlled bymeans of the temperature of the etchant, the concentration of nitricacid in the etchant, or the like.

In accordance with another preferred aspect of the present invention,the etching method comprises the steps of forming a metal film on achromium film formed on a substrate, forming a resist having apredetermined pattern on the metal film, etching the metal film and thechromium film using such a first etchant with which both the chromiumfilm and the metal film are etched, and the etching rate to the metalfilm is higher than the etching rate to the chromium film, and removingthe metal film using a second etchant after removal of the resist. Inthis aspect, the metal film is formed on the chromium film and then, themetal film and the chromium are etched using such a first etchant thatboth the chromium film and the metal film are etched, and the etchingrate to the metal film is higher than the etching rate to the chromiumfilm. Therefore, during etching using the first etchant, the metal filmis etched faster than the chromium film. As a result, etching isperformed while decreasing the contact area of the chromium film withthe metal film. Therefore, the degree of etching of the chromium film islarger as the position of the chromium film comes closer to the metalfilm, so that the etching end surface of the chromium film is inclined,to have a tapered shape.

In a preferred manner of this aspect, an aluminum film or an aluminumalloy film is selected as a metal film. When the aluminum film or thealuminum alloy film is used as the metal film, phosphoric acid can beused as a first etchant. If the phosphoric acid is used, both thechromium film and the aluminum film are etched, and the etching rate tothe aluminum film or the aluminum alloy film becomes higher than theetching rate to the chromium film. In addition, if phosphoric acid isused as the first etchant, a residue of the chromium film remains on thesubstrate after etching. The residue of the chromium film can be removedby etching using a chromium etchant which is a third etchant.

According to the etching method of the present invention, a side surfaceof the chromium film can be tapered in a simple process, so that themanufacturing cost can be reduced, as compared with the conventionalmethod. Furthermore, the chromium film can be etched such that the sidesurface thereof has a desired angle by selecting the temperature, theconcentration and the type of the etchant, so that etching can beperformed with a high precision. Thus, etching can be performed inreproducible manner.

These objects and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view showing a state before etching of achromium film according to one example of a conventional method;

FIG. 1B is a cross sectional view showing a state after etching of thechromium film according to one example of the conventional method;

FIG. 1C is a cross sectional view showing a state after removal of aresist according to one example of the conventional method;

FIG. 2A is a cross sectional view showing a state in which a resist isremoved after first etching of a chromium film according to anotherexample of the conventional method;

FIG. 2B is a cross sectional view showing a state in which the resist isformed on the chromium film after the first etching according to anotherexample of the conventional method;

FIG. 2C is a cross sectional view showing a state in which secondetching of the chromium film is performed to remove the resist accordingto another example of the conventional method;

FIG. 3A is a cross sectional view showing a state in which a resistpattern is formed on a chromium film in one embodiment of the presentinvention;

FIG. 3B is a cross sectional view showing a state in which the chromiumfilm is etched while stripping off the resist pattern from the chromiumfilm according to one embodiment of the present invention;

FIG. 3C is a cross sectional view showing a state after removal of theresist pattern in one embodiment of the present invention;

FIG. 4 is a diagram showing the relation between the concentration ofnitric acid in an etchant and an angle of an etching end surface of thechromium film;

FIG. 5A is a cross sectional view showing a state in which a metal filmis formed on a chromium film and a resist is formed thereon in anotherembodiment of the present invention;

FIG. 5B is a cross sectional view showing a state in which the chromiumfilm and the metal film are etched using a first etchant in anotherembodiment of the present invention;

FIG. 5C is a cross sectional view showing a state in which a residue ofthe chromium film is removed using a third etchant in another embodimentof the present invention;

FIG. 5D is a cross sectional view showing a state in which the metalfilm is removed using a second etchant in another embodiment of thepresent invention; and

FIG. 6 is a diagram showing the relation between the etching rate in thecase in which concentrated phosphoric acid is used as a first etchantand the temperature of the etchant in another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3A, 3B and 3C are cross sectional views for explaining a firstembodiment of the present invention. FIG. 3A shows a state in which aresist pattern is formed on a chromium film in the first embodiment ofthe present invention. In FIG. 3A, a chromium film 22 having a thicknessof 500 to 5000 Å is formed on a silicon substrate 21 by, for example, aplating process. A resist pattern 23 having a thickness of 0.5 to 3 μmis formed on the chromium film 22 using, for example, a conventionalphotolithographic technique. A photoresist OFPR-77E (manufactured byTOKYO OHKA KOGYO CO., LTD.) which is a resist film having a phenolnovolak resin as a principal chain is employed as the resist.

The substrate 21 is then dipped in an etchant at a temperature of 45° C.having the composition of 17 g of ammonium cerium (IV) nitrate, 13 cc ofnitric acid and 87 cc of water. At this time, the resist pattern 23starts to be stripped off from an end thereof by nitric acid in theetchant, as shown in FIG. 3B. In addition, the chromium film 22 isoxidized by an oxidizing agent in the etchant, to dissolve in theetchant as ions, so that etching proceeds. Since this etching andstripping of the resist pattern 23 proceed simultaneously, the etchingend surface of the chromium film 22 is tapered, as shown in FIG. 3B.

Finally, the resist pattern 23 is removed, as shown in FIG. 3C.

FIG. 4 is a diagram showing an example of the relation between theconcentration of nitric acid in the etchant and an angle of the etchingend surface of the chromium film. As shown in FIG. 4, the higher theconcentration of nitric acid in the etchant is, the smaller an angle ofthe etching end surface of the chromium film becomes. In addition, thehigher the temperature of the etchant is, the smaller the angle of theetching end surface of the chromium film becomes. Thus, the higher thetemperature of the etchant is, the lower the concentration of nitricacid can be. Furthermore, the angle of the etching end surface of thechromium film can be controlled by changing the temperature of theetchant and the concentration of nitric acid in the etchant.

Although in the above described first embodiment, a silicon substratewas used as a substrate, a substrate of another material can be used.Although in the above described first embodiment, the chromium film wasformed using a plating technique, the chromium film may be formed usinga dry film forming technique such as a vacuum evaporation process.Although in the above described first embodiment, the photoresistOFPR-77E was used as a resist, another macromolecule having a phenolnovolak resin as a principal chain may be used. In addition, anotherresin can be also used as the resist, which can be stripped off from thechromium film using an etchant during etching.

Although in the above described first embodiment, a photolithographictechnique is used as a method for forming the resist in a predeterminedpattern, another technique such as a printing technique may be used. Inaddition, an etchant of any other composition may be used, which canstrip off the resist from the chromium film during etching. Although inthe above described first embodiment, a dipping technique is used as amethod for etching the chromium film, another technique such as ashowering technique may be used.

FIGS. 5A, 5B, 5C and 5D are cross sectional views for explaining asecond embodiment of the present invention. In FIG. 5A, a chromium film32 having a thickness of 500 to 2000 Å is formed on a silicon substrate31 by, for example, a vacuum evaporation process. An aluminum film 33having a thickness of 1000 to 20000 Å is formed on the chromium film 32by, for example, the vacuum evaporation process. A resist pattern 34formed by, for example, a photoresist OFPR-800 (manufactured by TOKYOOHKA KOGYO CO., LTD.) is formed on the aluminum film 33.

The aluminum film 33 and the chromium film 32 are then etched usingphosphoric acid, for example, concentrated phosphoric acid at 45° C.Since both the chromium film and the aluminum film are etched, and theetching rate to the chromium film 32 is lower than the etching rate tothe aluminum film 33, an etching end surface of the chromium film 32 istapered, as shown in FIG. 5B. FIG. 6 is a diagram showing the relationbetween the etching rate to a two-layer film and the temperature of anetchant.

In such etching using phosphoric acid, a residue 32a of the chromiumfilm remains on the substrate 31. The residue 32a of the chromium filmcan be removed using a chromium etchant, for example, HClO₄ /(NH₄)₂Ce(NO₃)₆ /H₂ O. FIGS. 5C shows a state after removing the residue 32a ofthe chromium film. The resist pattern 34 is then removed by, forexample, plasma ashing. The aluminum film 33 is then removed using asolution of phosphoric acid containing nitric acid, such as a solutionat 45° C. having a volume ratio of phosphoric acid:nitric acid=20:1,which state is shown in FIG. 5D. In the above described manner, thechromium film 32 can be etched such that the etching end surface thereofis tapered.

Although in the above described second embodiment, the silicon substrateis used as a substrate, a substrate of material such as a glasssubstrate or the like can be used. Although in the above describedsecond embodiment, the chromium film is formed by a vacuum evaporationprocess, the chromium film may be formed by a film forming process suchas a sputtering process and a plating process. Although in the abovedescribed second embodiment, the aluminum film is used as a metal film,an aluminum alloy film may be used. In addition, the aluminum film andthe aluminum alloy film may be formed by not only the vacuum evaporationprocess but also a film forming process such as the sputtering process.Although in the second embodiment, the photoresist OFPR-800 is used as aresist, another resist can be used. Although in the second embodimentHCl₄ O₄ /(NH₄)₂ Ce(No₃)₆ /H₂ O is used to remove a residue of thechromium film, another chromium etchant may be used. Furthermore,although the resist is removed by plasma ashing, another resist removingprocess may be employed. Although concentrated phosphoric acid at 45° C.is used as a first etchant, another etchant can be used in which boththe chromium film and the aluminum film are etched, and the etching rateto the aluminum film which is a metal film is higher than the etchingrate to the chromium film.

An angle of a tapered shape formed on the etching end surface of thechromium film can be controlled to a desired angle by suitably selectingthe material of the metal film and the type, the temperature or the likeof the first etchant.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method for etching a chromium film formed on asubstrate, comprising the steps of:(a) providing a chromium film formedon a substrate, (b) forming a coating film having a predeterminedpattern on the chromium film, to selectively expose the chromium film,and (c) etching the exposed chromium film with an etchant comprising atleast 2 mol/l of nitric acid, the concentration of nitric acid in theetchant and the temperature of the etchant being sufficient to cause adecrease in the contact area of the chromium film with the coating film,whereby sidewall surfaces of a remaining pattern of the etched chromiumfilm are tapered.
 2. The method according to claim 1, wherein thecoating film comprises a resist and the concentation of nitric acid inthe etchant and the temperature of the etchant are sufficient to strip aportion of the resist from the chromium film while etching the chromiumfilm.
 3. The method according to claim 2, wherein the resist comprises aphenol novolak resin as a principal chain.